convert: print darcs error messages iff --debug

hgext/convert/common.py

@@ -228,7 +228,9 b' class commandline(object):'
             except TypeError:
                 pass
         cmdline = [util.shellquote(arg) for arg in cmdline]
-        cmdline += ['2>', util.nulldev, '<', util.nulldev]
+        if not self.ui.debug:
+            cmdline += ['2>', util.nulldev]
+        cmdline += ['<', util.nulldev]
         cmdline = ' '.join(cmdline)
         return cmdline